Negative resistance circuits



Oct, 21, 1930. F. H. GRAHAM 1,779,126

NEGATIVE RES ISTANCE CIRCUITS Filed Aug. 1, 1927 Arron/v5) fiPAN/f HUM/4H0 ERA/1AM I Patented Oct. 21, 1930 UNITED STATES PATENT OFFICE FRANK HOWARD GRAHAM, OF BROOKLYN, NEW YORK, ASSIGNOR TO BELLTELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 01' NEW YORK Application filed August 1, 1927. Serial No. 209,753.

This inventionrelates to a method of and means for obtaining negative resistance and particularly .to a method and means for obtaini'ng a negative resistance utilizing space discharge devices.

In the prior art, circuits using vacuum tube amplifiers for securing negative resistances have been known forsome time. Various vacuum tube circuits for this purpose are shown, for example, in an articleen'titled .Resistance neutralizationan application of thermionic amplifier circuits, by Bennett and Peters, pages 234 to 248 of the J ournal of the A. I. E. E. vol. 41 (1922). These 1 circuits, however, as they all utilize inductive feed-back -arrangements for this purpose, produce a negativeresistance which is a function of frequency. 5

In accordance with the present invention a new circuit arrangement for obtaining negative resistance which is independent of frequency over a wide range of frequencies is provided. This circuit arrangement in a preferred. embodiment comprises an amplifier.circuit having two vacuum tube stages, resistance-coupled, and a feed-back resistance coupling-between the output circuitof the last stage and the input circuit of the first stage. In. this circuit, between terminals con nected respectively to the grid of the first tube and the plate'of the last tube, a negative resistance is obtained which is a function ofthe constants of the circuit and which may be expressed in terms of the plate circuit 3 resistance of the last tube, the resistance .in the input circuit of the first tube and the net voltage amplification of the amplifier circuit, The negative resistance ,thus'obtained is advantageousin'that it remains stable over I a wide range of frequencies, and may easily be made-to vary between widely different limiting values by suitably adjusting the elements of the circuit.

The various features andobjects of the 1nv vention will'be better understood from the following detailed'description thereof when read in connection with the accompanylng drawing, in which:

Fig. 1 shows diagrammatically one em 6 bodiment of the circuit of the invention as 7 ance with the invention for obtaining a negative resistance, comprising a resistancecoupled amplifier circuit of two stages, the first stage comprising a space discharge device -1 having a cathode or a filament 3,an anode or plate 4, and a control electrode or grid 5, and the second stage comprising a space discharge device 2 having a cathode or filament 6, an anodeor plate 7 and a control electrode or grid 8. The cathodes, 3 and 6, respectively, of tubes 1 and 2 are connected together and may be heated by a common battery 9. The output circuit of tube 1 and the input circuit of tube 2 are coupled through coupling resistance 10, common to the two circuits. The space current for tube 1 is supplied by battery 11 through coupling resistance 10 to the plate 4 of tube 1. Similarly, space current for tube 2 is supplied by battery 11 through battery feed resistance 13 to the plate 7 of the tube 2. The grid potentials of tubes 1 and 2 respectively, are adjusted to any suitable values by the grid batteries 14- 'and 15 in the respective input circuits of the tubes. The grid 5 of tube 1 is connected through grid battery 14 and resistance R v to cathode 3 of tube 1, and through grid battery 14, resistance R variable resistance R5,

conductor 16, and battery 17 to the plate 7 of tube 2. Battery 17 is poled so as to oppose plate battery 11 and its potential is adjusted so that for the condition in the circuit, when no external voltage is impressed upon resistances R and R there is no current flow- .ing in conductor 16. Terminals 18 and 19 diagrammatically an electrical circuit22, in.

'sents the net positive resistance of circuit 22. lVhen the terminals 20 and 21 are joined through a suitable electric circuit, the generator 23 drives a current in the direction indicated by the arrow through the resistance R,, the magnitude of which is dependent on the value of the resistance in the circuit. It is desired to insert in the electrical circuit across the terminals 20 and 21 a network which will. act as a negative resistance, that is, act in a sense to effectively neutralize the effect of a part of the positive resistance R in the circuit so that the current flowing therein will be larger than the current which is produced by the generator 23 alone. In accordance with the invention, this result is accomplished by connecting the terminals 18 and 19 of the amplifier circuit by suitable means to the terminals 20 and 21 of circuit 22, which provides a negative resistance between those terminals which is a function only of the constants of the amplifier circuit,

i. e. of the value of the elements R R the anode to cathode resistance of tube 2 and the net voltage amplification of the amplifier circuit, and is independent of the value of the electromotive force produced by generator 23 in the circuit. By adjusting the values of the elements referred to, it has been found that a negative resistance of any desired value may be obtained, this negative resistance being independent of frequency over a nude range of frequencies and also independent of the external circuit electromotive force and impedance.

In order that the negative resistance thus obtained should .be stable and free from so called singing effects, it is necessary. as will be explained below, to select the elements of the circuit so that the negative resistance is at all times less than the positive resistance which it faces, that is, so that the net impedance of the circuit as a whole is positive.

The operation of the circuit of Fig. 1 will i now be described: Let it be assumed that the alternating current produced by generator 23 will flow through the resistances R R and R in a direction indicated by the arrow. The drop in potential in resistance R due to the current flowing through it will cause a decrease in the negative voltage applied to the grid 5 of tube 1 and a resultant increase in the average current flowing through resistance 10 in the output circuitof tube 1. The resultant drop in potential in the resist ance 10 will cause an increase in the negative potential; applied to grid 8 of tube 2, and thus a decrease'in the average current in the output circuit of the tube which due to the impedance thereto ofiered by resistance 13 will be fed back through conductor 16 in the direction indicated. by the dotted arrow to electrical circuit 22 and will be in phase with the current produced in that circuit by the generator 23. The magnitude of this superposed current will depend on the value of the resistance R in that circuit, and a maximum value for this current will be obtained by making R infinite. It may be desired, however, to limit the amount of superposed current, and this may be done by adjusting resistance R to a suitable value.

By inserting the circuit arrangement shown to the right of the dot-dash line AA in circuit 22, therefore, the current flowing in the latter circuit may be increased without necessitating an increase in the potential of generator 23. It appears, therefore, thatthe network inserted between terminals 20' and 21 of circuit 22 will act as a negative resistance, as in effect neutralizes a part of the positive resistance in circuit 22.

The theory on which the invention is based will be clear from the following explanation in connection with Fig. 2, which shows a simplified representation of the circuit of. Fig. 1. In Fig. 2, the elements R R and R correspond to the similarly characterized elements in the circuit of Fig. 1; R represents the A. C. plate circuit resistance of the space discharge device 2'; E represents the voltage of the generator 23; M'represents the net voltage amplification for the circuit of Fig. 1 from the grid of tube 1 into the plate circuit of tube 2; MR zl represent the voltage reproduced in the anode circuit of tube 2 due to M, R and 5 where 5 is the current driven through the circuit by the voltage E of generator 23; and I is the resultant current iiowing through the resistance lR- due to the voltage E and the various electromotive forces in the circuit of Fig. l'to the right of the dotdash line.

The resultant current ll may be represented in the form of a series,

r=a+a+an where the current produced in the electrical circuit due to the voltage E is Then the current produced inthe circuit due to the potential drop produced by current '5 in R is Then the current produced in the circuit due to the potential drop produced by current i in R is and the current produced in the circuit due, to MR i 1s The summation of the series is n. 1'l0: p (9) must be 1. (8)

where r=ratio of terms, a=first term and Z==ar"' =last term. Where a co Let R =negative resistance across the ter- For maximum I, R 00, then R =R +R,,MR =R,,+ (1M)R'- (14) prevent the circuit from developing singing tendencies, the elements should be so selected as shown by Equation so that the ratio fi@ m R,,(R +R )+R (R +R The circuit of the invention as described above produces a negative resistance which is substantially independent of frequencies since its value is determined largely, as shown by the above equation, by elements which are substantially pure resistances, which circuit is within wide limits stable and non-oscillatory, and which may be adjusted Within wide limits.

The system above illustrated and described should be construed merely as typical and not as limiting the invention, the'scope of which is defined in the appended claims.

hat is claimed is:

1. An electrical network forsecuring a negative resistance which is independent of frequency, comprising a resistance coupled amplifier having an even number of stages each including a space discharge device having an input and an output circuit, each de vice having a cathode, an anode and a control electrode, a common source of space current for both of said devices, a resistance connected in the input. circuit of the first space discharge device in said amplifier between the control electrode and the cathode thereof, a second resistance connected in the output circuit of the last space discharge device in said amplifier between the anode thereof and said'source, a terminal connected to a point in the input circuit of said first device between the control electrode thereof and said resistance in said input cir cuit, a second terminal, a connection between said second terminal and a point in the output circuit of said last space discharge device between the anode thereof and said second resistance, and means in said connection allowing transmission therethrough of the amplified current variations while preventing transmission therethrough of direct current from the output circuit of said last device,-said network producing between the first mentioned terminal and said second terminal when an external electromotive force is applied therebetween a. negative resistance which is a function solely of the constants of the amplifier circuit and independent of the value ofsaid external electromotive force.

2. In combination, an amplifier circuit comprising two space discharge devices, each having an input and an output circuit, and each having a cathode, anode and a control electrode, the output circuit of one of said devices being coupled to the input circuit of the other of said devices through a resistance, a single source of space current for both of said devices, a second resistance connect-' ed in the input circuit of said one device between the control electrode and anode thereof, a third resistance connected in the output circuit of said other device between the anode thereof and said source, a terminal connected to the input circuit of said one device between the control electrode thereof and said one resistance, a second terminal, and a conductor connecting said second terminal to the output circuit of said other device between the anode thereof and said third resistance. and means in said conductor for preventing flow in said conductor of direct current from the output circuit of said other device.

3. In combination, an amplifier circuit comprising two space discharge devices, each having an input and an output circuit, and each having a cathode, anode and a control electrode, the output circuit of one of said devices being coupled to the input circuit of the other of said devices through a resistance, a common source of space current for said devices, a second resistance connected in the input circuit of said one device between the control electrode and anode thereof, a third resistance connected in the output circuit of said other device between the anode thereof and said source, a terminal connected to the input circuit of said one device between the control electrode thereof and the first-mentioned resistance, a second terminal, a conductor of low resistance to alternating currents connecting said second terminal to the output circuit of said other device between the anode thereof and said third resistance, and means in said conductor for preventing flow in said conductor of direct current from the output circuit of said other device, said means comprising a source of direct current electromotive force poled so as to oppose the action of the common source of space current for said devices.

4. In combination, a two stage amplifier circuit comprising two space discharge devices, each having an input and an output circuit, and a cathode, anode and control electrode, the output circuit of one of said devices being coupled to the input circuit of the other of said devices through a resistance, a single source of space current for both of said devices, a second resistance connected between the control electrode and the cathode of said one device, a terminal connected bet-ween said second resistance and the control electrode of said one device, a third resistance connected in the output circuit of said other device between the anode thereof and said single source of space current, a second terminal, a conductor connecting said second terminal to a point in said output circuit for said other device between said third resistance and the anode of said other device, and means in said conductor offering a high resistance to flow therein of direct current from the space current source in the output circuit of said last device and low resistance to flow of the amplified current variations from said out-put circuit.

5. An electrical network for securing a negative impedance between two terminals of an electrical circuit including a source of alternating current waves, said network comprising a resistance coupled amplifier having an even number of stages each including a space discharge device having a cathode, an anode and a control electrode, and eachhaving an input and an output circuit, a common source of space current for both devices, an impedance in the input circuit of the first discharge device in said amplifier connected between the cathode and control electrode thereof, a connection from one of said terminals to a point in the input circuit of said first discharge device between the control electrode thereof and said impedance, a connection between the other of said terminals and a point in theoutput circuit of the last space discharge device in said amplifier between the anode thereof and said source of alternating current waves, and means in the last-mentioned connection for preventing direct current from the output circuit of said last space discharge from being transmitted to said electrical circuit.

In witness whereof, I hereunto subscribe my name this 28th day of July, A. D. 1927.

. FRANK HOWARD GRAHAM. 

